Massive explosion may have involved large amounts of ammonium nitrate stored at the site. It has caused some of the worst industrial accidents in U.S. history.

The blast at a West, Texas, fertilizer plant on Wednesday night was so massive that investigators believe it probably involved a significant amount of ammonium nitrate, a chemical that some scientists say should be regulated as an explosive.

In a report filed with the Texas Department of State Health Services on Feb. 26, West Fertilizer Co. said that it had up to 270 tons of ammonium nitrate at its facility, along with up to 100,000 pounds of liquid ammonia. The exact amounts on hand at the plant are not yet known, officials said.

Pentagon explosives experts say that a detonation involving 270 tons of ammonium nitrate would be larger than almost any non-nuclear weapon possessed by the U.S.

Government officials close to the investigation are focusing on reports that a very large quantity of ammonium nitrate was being stored at the plant, which industry officials say was a blending and distribution operation that served local farmers. In its filings with state officials, the company acknowledged that it had a range of industrial chemicals that it said were "extremely hazardous."

The explosion occurred after a fire started at the facility. It flattened buildings several blocks away, reflecting the type of explosive force commonly associated with ammonium nitrate.

Company filings with federal regulators named Donald Adair as the owner of the plant. Adair and other company officials could not be reached Thursday.

In its dealings with Texas regulators, the company said any accident would not be large enough to cause an explosion. A risk management plan filed by the company in 2011 made no mention of ammonium nitrate being stored at the facility.

Five years earlier, the U.S. Environmental Protection Agency fined the company $2,300 for not even having a risk management plan in place and for other problems, including poor employee training and lack of a formal maintenance plan.

Last year, the company agreed to pay a $5,250 fine to the Pipeline and Hazardous Materials Safety Administration for storing ammonia in improperly marked tanks and for transporting the material without a security plan.

About 8 billion pounds of ammonium nitrate is produced annually in the U.S., with half going to the agriculture industry and the other half to the explosives industry.

The chemical was also used in the 1995 Oklahoma City bombing, the first attack on the World Trade Center in 1993 and the bombings of two U.S. embassies in Africa in 1998. Attempts to tighten regulation of the material have been bogged down since the early 1990s.

A Pentagon explosives expert said that government testing of ammonium nitrate has proven its deadly potential. The chemical generates a slow-moving but very high-pressure blast that causes significant organ damage to humans.

The fertilizer industry has fought tighter controls on the material, arguing that it is not explosive in the concentrations sold in retail stores. Farmers use the material to blast stumps out of the ground.

But a series of tests in New Mexico demonstrated that even low-level concentrations of ammonium nitrate, common in fertilizer sold at home improvement stores, could generate serious explosions.

"It is a very significant explosive force," the military expert said.

Ammonium nitrate explosions have caused some of the worst industrial accidents in U.S. history, including a 1947 disaster in Texas City that killed more than 500 people and injured 5,000. Accidental explosions occur regularly around the world, including incidents that killed 37 in Mexico in 2007.

Neal Langerman, principal chemist with Advanced Chemical Safety, a San Diego industrial consulting firm, said the explosion did not necessarily involve ammonium nitrate. The initial fire at the plant could have caused a failure of the tanks containing ammonia gas, also known as anhydrous ammonia.

"If a tank containing a liquefied flammable gas is subjected to fire, that tank could fail catastrophically, releasing the gas, which will instantly ignite, causing a catastrophic explosion," he said.

The EPA and various state regulators are responsible for overseeing safety at the West Fertilizer plant. All facilities with hazardous substances on site must file a list of those substances with state and local authorities. A list filed by West Fertilizer on Feb. 26 showed that it had not only ammonium nitrate and anhydrous ammonia, but a variety of agriculture chemicals.

In its 2011 risk management plan, the company said that the anhydrous ammonia did not pose any threat of fire or explosion. "The worst-case release scenario would be the release of the total contents of a storage tank released as a gas over 10 minutes," the plan said.

The company did not have backup safety systems inside the facility, like auto shut-offs, alarms, or emergency air and power, according to the plan. It also did not have so-called mitigation systems, like sprinklers, fire walls and blast walls.

"Many people would be surprised to learn that there are no federal setback or buffer zone requirements to keep extremely hazardous chemicals away from nearby schools, homes, and businesses, and no evaluation requirements for companies to see if they can store lesser quantities or use safer chemicals," said Paul Orum, a Washington-based chemical safety consultant.

The disaster is being investigated by teams from five federal agencies, including the Bureau of Alcohol, Tobacco, Firearms and Explosives and the Environmental Protection Agency, among others.